Ubiquitin Links Smoothened to Intraflagellar Transport to Regulate Hedgehog Signaling

Abstract

In the absence of hedgehog ligand, patched-1 (Ptch1) localizes to cilia and prevents ciliary accumulation and activation of smoothened (Smo). Upon ligand binding, Ptch1 is removed from cilia, Smo is derepressed and accumulates in cilia where it activates signaling. The mechanisms regulating these dynamic movements are not well understood but defects in intraflagellar transport components including Ift27 and the BBSome cause Smo to accumulate in cilia without pathway activation. We find that in the absence of ligand-induced pathway activation, Smo is ubiquitinated and removed from cilia, and this process is dependent on Ift27 and BBSome components. Activation of hedgehog signaling decreases Smo ubiquitination, and ciliary removal, resulting in its accumulation. Blocking ubiquitination of Smo by an E1 ligase inhibitor or by mutating two lysine residues in intracellular loop three cause Smo to aberrantly accumulate in cilia without pathway activation. These data provide a mechanism to control Smo's ciliary level during hedgehog signaling by regulating the ubiquitination state of the receptor. SummaryHedgehog signaling involves the dynamic movement of receptors and effectors in and out of cilia. We find that the dynamics of Smo is regulated by ubiquitination, which regulates its interaction with the intraflagellar transport system to control ciliary levels of this receptor.

Mutations in GRK2 cause Jeune syndrome by impairing Hedgehog and canonical Wnt signaling

“…GRK2 was shown to phosphorylate SMO C‐tail, leading to ciliary accumulation of SMO and Hh pathway activity (Chen et al , 2011); the former has been disputed and appears cell context dependent (Zhao et al , 2016; Pusapati et al , 2018). GRK2‐mediated SMO phosphorylation was shown to induce β‐arrestin (ARRB) recruitment, leading to KIF3A‐dependent cilia accumulation of SMO (Chen et al , 2004; Kovacs et al , 2008); however, later studies showed normal cilia accumulation of SMO in ARRB knock‐out cells (Pal et al , 2016; Desai et al , 2020). Ligand‐dependent removal of the Hh pathway inhibitor GPR161 was shown to depend on GRK2 activity (Pal et al , 2016; Pusapati et al , 2018).…”